1. Field of the Invention
This invention relates to an image sensing apparatus using plural light sensors, and more particularly to an image sensing apparatus with shading compensation in that variations in the sensitivity of the light sensors and variations in the intensity of illumination are compensated.
2. Description of the Prior Art
Hitherto, an image sensing apparatus with shading compensation has been described in Japanese Patent Application No. 54-32013. In this apparatus, both black-level signals and white-level reference signals produced from light sensors are stored in a memory. The shading compensation is performed by the use of these stored signals. The black level signals correspond to output signals of the light sensors when no light is input thereto. This state can be readily created by simply turning off the light source, or by covering the light sensors whereby substantially no light may be introduced. Thus, the black level signals can be obtained relatively easily. Conversely, in order to produce white level reference signals, a white reference board having a uniform reflection factor is necessary. However, an ideal reference board is difficult to obtain. Nevertheless, an original document sensing scanner or the like generally has a white reference board incorporated within an image sensing system. By using the board, the white level reference signals can be obtained selectively as data for shading compensation. These data can be used to compensate for variations in brightness, such as light-quantity distribution noise that varies over time, called shading noise. However, such a white reference board occasionally becomes dirty. When the board is dirtied, this dirtiness becomes indistinguishable from variations in sensitivity of the light sensors. Thus, shading compensation cannot be achieved. Further, in a camera used in an enclosed area, such as the human stomach or the like, it is practically impossible to obtain uniform white reference signals necessary to perform shading compensation because of foreign substances accumulating on the camera.
As described above, in the conventional image sensing apparatus with shading compensation, there have been problems in that uniform white reference signals are very difficult to obtain. Thus, it is difficult to distinguish between variations in sensitivity of the light sensors and variations of white reference signals. As a result of this, proper shading compensation cannot be achieved consistently.